The active site of a number of metalloproteins contains a binuclear copper unit designated as "type 3" or EPR-nondetectable copper which exhibits unusual spectroscopic properties and interacts with oxygen as part of its biological function. This unit functions to bind oxygen (hemocyanin), to act as a two electron oxidase (tyrosinase, laccase, and ceruloplasmin) and to hydroxylate organic substrates (tyrosinase). A series of protein derivatives have been prepared which allows the "type 3" copper active site to be systematically varied for spectroscopic study using a number of appropriate techniques. These studies have generated an effective picture of the oxyhemocyanin active site, provided a structural interpretation of the unusual spectroscopic properties and have demonstrated strong similarities between the active site in hemocyanin and tyrosinase. Chemical and spectroscopic studies are now proposed which should develop a detailed understanding of the properties of the "type 3" copper active site; how it activates oxygen and interacts with organic substrates and how changes in site structure may contribute to inter subunit cooperative interactions and to variation in biological function.